I'd second that - probably chain and sprockets. After the vibes start the chain usually starts to 'clank' as one or more links develop excessive side play. I usually take the clanking as a sign to change them ASAP - I had a chain snap at this stage, and it cause a fair bit of damage. Before you check anything further, check to see how much sideways play you have on your chain.
There are lots of other things it could be, though. Sometimes tyres wear strangely and cause odd vibes. Did you fit tyres you haven't tried before?
Tall men see everyone else's bald patch but their own.......